Actuator and micromirror for fast beam steering and method of fabricating the same

Abstract

A micromirror for fast beam steering and method of fabricating the same. The micromirror of the present invention is lightweight and optically flat, and includes a tensile membrane that is stretched under high tension across a rigid single-crystal silicon support rib structure. A thin layer of gold may be deposited on the polysilicon membrane to improve reflectivity. The tensile stress in the membrane gives the micromirror a very high resonant frequency, thereby allowing the mirror to be scanned at high frequencies without exciting resonant nodes that may compromise the flatness of the optical surface and ruin its optical properties. The tensile stress also causes the optical surface to be stretched flat. The micromirror of the present invention may be actuated by a staggered torsional electrostatic combdrive.

What is claimed is:  
     
         1 . A micromirror, comprising: 
 a rigid support rib; and    a membrane stretched across and fixedly mounted on the rigid support rib to form an optical surface of the micromirror.    
     
     
         2 . The micromirror of  claim 1 , wherein the rigid support rib comprises a substantially circular member and wherein the optical surface of the micromirror comprises a substantially circular shape.  
     
     
         3 . The micromirror of  claim 2 , wherein the optical surface has a diameter between 500 μm and 2000 μm.  
     
     
         4 . The micromirror of  claim 1 , wherein the rigid support rib comprises single-crystal silicon.  
     
     
         5 . The micromirror of  claim 1 , wherein the membrane comprises a layer of annealed polysilicon.  
     
     
         6 . The micromirror of  claim 5 , wherein the layer of annealed polysilicon has a thickness of between approximately 0.5 μm and 1 μm.  
     
     
         7 . The micromirror of  claim 5 , wherein the membrane further comprises a reflective film covering the optical surface.  
     
     
         8 . The micromirror of  claim 7 , wherein the reflective film comprises a tensile metallic film.  
     
     
         9 . The micromirror of  claim 8 , wherein the tensile metallic film comprises gold.  
     
     
         10 . The micromirror of  claim 7 , wherein the tensile metallic film comprises aluminum.  
     
     
         11 . The micromirror of  claim 10 , wherein the tensile metallic film comprises silver.  
     
     
         12 . The micromirror of  claim 1 , wherein the membrane comprises a layer of annealed silicon nitride.  
     
     
         13 . The micromirror of  claim 1 , wherein the membrane comprises a layer of tensile metallic film.  
     
     
         14 . The micromirror of  claim 1 , wherein the membrane comprises a layer of tensile plastic film.  
     
     
         15 . The micromirror of  claim 1 , wherein the rigid support rib has a height of between approximately 20 μm and 200 μm.  
     
     
         16 . The micromirror of  claim 2 , wherein the rigid support rib has a width of between approximately {fraction (1/40)} and {fraction (1/10)} of a diameter of the substantially circular member.  
     
     
         17 . The mircormirror of  claim 1 , wherein the optical surface has a tensile stain between approximately 50 MPa and 1 GPa.  
     
     
         18 . A method of fabricating a micromirror, comprising: 
 bonding a first wafer to a second wafer to form a sandwich including the first wafer, an oxide layer, and the second wafer;    etching the second wafer to expose an first portion of the oxide layer;    depositing a polysilicon layer on the first exposed portion of the oxide layer;    depositing a protective oxide layer on the polysilicon layer;    annealing the polysilicon layer to yield a pre-determined tensile stress therein;    etching the second wafer to form a support rib structure after depositing the protective oxide layer;    etching a backside of the first wafer to expose a second portion of the oxide layer; and    removing the exposed second portion of the oxide layer to release the micromirror.    
     
     
         19 . The method of  claim 18 , further comprising sputtering a reflective material on the polysilicon layer.  
     
     
         20 . The method of  claim 19 , wherein the sputtering comprises sputtering a film of gold on the polysilicon layer.  
     
     
         21 . The method of  claim 19 , wherein the sputtering comprises sputtering a film of silver on the polysilicon layer.  
     
     
         22 . The method of  claim 19 , wherein the sputtering comprises sputtering a film of aluminum on the polysilicon layer.  
     
     
         23 . A method of fabricating a micromirror, comprising: 
 bonding a first wafer to a second wafer to form a sandwich including the first wafer, an oxide layer, and the second wafer;    etching the second wafer to expose an first portion of the oxide layer;    depositing a silicon nitride layer on the first exposed portion of the oxide layer;    depositing a protective oxide layer on the silicon nitride layer;    annealing the silicon nitride layer to yield a pre-determined tensile stress therein;    etching the second wafer to form a support rib structure after depositing the protective oxide layer;    etching a backside of the first wafer to expose a second portion of the oxide layer; and    removing the exposed second portion of the oxide layer to release the micromirror.    
     
     
         24 . The method of  claim 23 , further comprising sputtering a reflective material on the silicon nitride layer.  
     
     
         25 . The method of  claim 24 , wherein the sputtering comprises sputtering a film of gold on the silicon nitride layer.  
     
     
         26 . The method of  claim 24 , wherein the sputtering comprises sputtering a film of silver on the silicon nitride layer.  
     
     
         27 . The method of  claim 24 , wherein the sputtering comprises sputtering a film of aluminum on the silicon nitride layer.